Method and apparatus for controlling plate thickness in a rolling mill

ABSTRACT

Method and apparatus for controlling plate thickness in a rolling mill, particularly in a cold rolling mill in which rolls are applied with at least one roll separating force acting in the direction opposite to the direction of roll depressing force to obtain a favourable crown shape of a rolled material. A rolled plate thickness (h) at the outlet side is exactly obtained, under a condition in which the roll separating forces are applied, from an equation H So + Q/M + (P - Q)/K wherein Q represents the actual measurement of the roll separating force; P represents the actual measurement of the roll depressing force; K represents the modulus of rigidity of the whole mill; M represents the modulus of rigidity of the rolls and the housing; and So represents the adjusted value of roll clearance, TO THEREBY CONTROL THE PLATE THICKNESS BY ADJUSTING THE ROLL SEPARATING FORCES AND THE ROLL DEPRESSING FORCE IN ACCORDANCE WITH VALUES CALCULATED FROM THE EQUATION.

llnited States Patent Kitanosono et a1.

[ Dec. 24, 1974 METHOD AND APPARATUS FOR CONTROLLING PLATE THICKNESS IN A ROLLING MILL [75] Inventors: Hidehiro Kitanosono; Takeaki Kubo; Shigeru Shida, all of Hitachi, Japan [73] Assignee: Hitachi Ltd., Tokyo, Japan [22] Filed: Nov. 27, 1972 [21] Appl. No.: 309,923

Related US. Application Data [63] Continuation of Ser. No. 60,223, Aug. 3, 1970,

' abandoned.

' [30] Foreign Application Priority Data Aug. 4, 1969 Japan 44-61009 [52] US. Cl. 72/8, 72/16 [51] Int. Cl B2lb 37/00 [58] Field of Search 72/6-12, 16,

[56] References Cited UNITED STATES PATENTS 3,318,124 5/19 67 Plaisted 72/8 3,394,566 7/1968 OBrien 72/8 3,461,705 8/1969 Neumann 72/243 3,496,744 2/1970 Mizuno et a1 72/12 3,531,960 10/1970 Stone 72/8 Primary Examiner-Milton S. Mehr Attorney, Agent, or FirmCraig & Antonelli [57] ABSTRACT Method and apparatus for controlling plate thickness in a rolling mill, particularly in a cold rolling mill in which rolls are applied with at least one roll separating force acting in the direction opposite to the direction of roll depressing force to obtain a favourable crown shape of a rolled material. A rolled plate thickness (h) at the outlet side is exactly obtained, under a condition in which the roll separating forces are applied,

4 Claims, 8 Drawing Figures COMPUTER Pmmrmnww 3355.830

' sum 1 95 9 FIG. 8

COMPUTER HESS/N0 /2 Mam/s DETECTOR CONTROL MEANS OPERATIO- NAL MEANS R K M M INVENTORS moemao kwmoswo, TAKEAKI kulso AND .smeERu smbfi Cwkdu Antoneui, s eulqrk H I ATTORNEYS METHOD AND APPARATUS FOR CONTROLLING PLATE THICKNESS IN A ROLLING MILL This is a continuation, of application Ser. No. 60,223 filed Aug. 3, 1970, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for controlling plate thickness in a rolling mill in which at least one roll separating force is applied between roll hearings to perform a rolling operation.

2. Description of the Prior Art Hithertofore, it has been known to apply at least one roll separating force between bearings of opposed rolls in order to prevent a rolled plate material from being formed into an undesirable shape due to the formation of roll crown. However, according to the prior control system, since variations of the plate thickness at the exit side of rolling stand due to the existence of the roll separating force has not been exactly controlled, the thickness of the rolled material cannot be maintained to a desired value when the shape of the plate is improved, or the shape of the plate cannot be sufficiently controlled when the plate thickness is maintained within an allowable range.

SUMMARY OF THE INVENTION An object of the present invention is to control the plate thickness by detecting, among other things, an effect of roll separating force, and to provide an apparatus which can exactly control the plate thickness maintaining a. good shape of the plate.

Another object of the present invention is to provide a plate thickness control means wherein roll separating force is positively utilized for controlling the plate thickness and wherein the response of control system is more rapid than in a plate thickness control in which only roll clearance is adjusted.

According to the aspect of the present invention, suitable roll separating force is applied between roll bearings, and the plate thickness at the exit side of roll stand under such a condition is obtained from an actual measurement of the roll separating force, an actual measurement of roll depressing force, the modulus of rigidity of rolls and housing, the modulus of rigidity of the whole mill and the roll clearance setting, the plate thickness thus obtained being compared with a desired plate thickness to control the thickness of a rolled plate.

According to another aspect of the present invention, when the plate thickness is controlled by using the plate thickness obtained as described above, a crown detector is provided at the inlet or exit side of the roll stand, the detected value being utilized to adjust the roll separating force for controlling the plate shape, the roll clearance being further adjusted in accordance with the adjustment of the roll separating force.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 through 5 show the principle of the present invention, wherein FIG. 1 is a diagrammatical view showing a plate being rolled;

FIG. 2 is a diagram showing a manner for obtaining the plate thickness in the rolling operation shown in FIG. 2;

F IG. 3 is a diagrammatic view showing roll separating force applied between opposed roll bearings;

FIG. 4 is a diagram showing the plate thickness at the exit side under conditions with and without roll separating force applied between roll bearings;

FIG. 5 shows, in a diagrammatical form, a resilient system of rolling means;

FIG. 6 is a diagrammatical view showing one embodiment of the present invention;

FIG. 7 is a view similar to FIG. 6 but showing another embodiment of the present invention; and

FIG. 8 is a schematic diagram showing a further embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First of all, a theoretical equation utilized in the present invention will now be described. In a conventional rolling mill in which, as shown in FIG. 1, roll separating force is not applied between roll bearings, the plate thickness at the exit side can be expressed by the following equation.

h S P/K I. wherein:

h represents the plate thickness at the exit side of a rolling mill;

S represents the roll clearance setting which is equal to the roll clearance when a material is not inserted between rolls;

P represents roll depressing force, and

K represents the modulus of rigidity of the whole mill including rolls and a housing.

Further, the above equation (1) can be modified as follows:

In FIG. I, the thickness of material (H) before rolling, the thickness h of a rolled plate and the roll depressing force P has, as previously known, the following relation.

V R, Q 'r a. wherein:

K represents an average value of deformation resistance of the material;

b represents the width of the material;

R represents the effective work roll radius which is determined by taking the roll flatness into account; and,

Q represents the correction factor.

Thus, when the clearance between work rolls is adjusted to S before a material is introduced therebetween, and a material of thickness H is introduced, the plate thickness after rolling operation h can be obtained from the equations (2) and (3). In FIG. 2 in which curves of the equations (2) and (3) are drawn, the thickness h can be obtained at the intersection of the two curves.

Now, assume that, as shown in FIG. 3, roll separating force is applied between roll bearings in the directions to force the rolls apart. In FIG. 3, the reference numeral I shows work rolls, 2 back-up rolls, 4 power apply means for applying work roll separating forces O between work roll bearings, 6 power apply means for applying back-up roll separating force 0,, between back-up roll bearings, and 13 a material being rolled. Q is roll separating force acting between the opposed work rolls, and O is roll separating force acting between the opposed back-up rolls. In FIG. 3, the roll clearance without roll separating force acting between the roll bearings and without any rolling material introduced between the rolls is defined as the roll clearance setting 5,. If the roll separating force is applied before a material is introduced between the rolls, the roll clearance S between the work rolls will become greater than the roll clearance setting 8,. This can be expressed by the following equation.

M represents the modulus of rigidity between the work roll bearing and the housing, and is the modulus of rigidity between the work roll bearing and the back-up roll bearing and of rigidity between the back-up roll bearing and the housing; and

M represents the modulus of rigidity between the back-up roll bearing and the housing.

Thus, it has been found that, when the roll separating force is applied between the opposed roll bearings, the thickness of the rolled plate h which may be expressed by the equation (2) will be practically increased be cause the roll clearance S in this equation will be essentially increased as apparent from the equation (4). This relation can be expressed by the following equation.

In the equation P represents a rolling pressure (reaction) which is directly applied on the material being rolled. This pressure P is different from the roll depressing force P detected at the back-up rolls, and can be expressed by the following equations.

From the equations (5) and (6), the thickness of the rolled plate 11 can be written as follows:

This equation (7) constitutes the basis of the present invention. In FIG. 4, the curve of the equation (7) is drawn in relation with the curves of the equation (2) and (3). In order to explain the equation (7) in more detail, the resilient system of the rolling mill of FIG. 3 in schematically shown in FIG. 5. In FIG. 5, K, represents the modulus of rigidity between the center of the material being rolled and the work roll bearing in which the rigidity of the material itself is included, K the modulus of rigidity between each work roll bearing and the cooperating back-up roll bearing, K the modulus of rigidity between each back-up roll bearing and the housing in which the rigidity of the housing itself is taken into account.

With initial roll clearance S, and when the material to be rolled is not introduced between the rolls and the forces Q and Q are applied, the following relation will be established because the reaction force P is zero.

P Qu' Q1:

In this instance, the displacement of the work roll will be a sum of deflections 8, and 8 of the portions between A and B, and B and C, respectively. The deflections 8 and 8 can be written as follows:

The variables in the equation (4) will thus be written as follows:

/MB 9. Further, as well known, the modulus of rigidity of the mill K can be expressed by the following equation.

That is, when the modulus of rigidity or spring constants K K and K are connected in series as shown in FIG. 5 and the force P is operated on each of the modulus of rigidity constants, the total change is represented as follows:

P/K1 P/K2 P/Ka Thus, the mill modulus of rigidity constant K in view of L can be represented by:

P/K P/Ki P/Kz 'l' P/KS which yields equation (10).

By introducing the relations of the equations (8), (9) and (10) into the equation (7), the rolled plate thickness h will be written as follows:

Referring now to FIG. 4, as will be noted in the equation (1 l the rolled plate thickness will be h which corresponds to the intersection a of the curves of the equations (2) and (3) when any of the forces Qw and 0,; is not applied, while the rolled plate thickness will be h which corresponds to the intersection B of the curves of the equations (3) and (7).

According to the present invention, the plate thickness can be detected with a high accuracy by using the aforementioned relationship to control the plate thickness.

An embodiment of the present invention will now be described taking reference to FIG. 6. In the drawing, the reference numeral 1 shows a pair of opposed work rolls, 2 back-up rolls, and 3 a work roll separating force detector such as a load cell for detecting a work roll separating force Q acting between the opposed work roll bearings. The numeral 4 shows force applying means such as a piston and cylinder for applying a separating force between the work roll bearings for controlling the shape of a rolled material. 5 shows a back-up roll separating force detector for detecting a back-up roll separating force Q; acting between the opposed back-up roll bearings, and 6 shows force apply means for applying a separating force between the back-up roll bearings for controlling the shape of the plate. The numeral 7 shows a roll depressing force detector for detecting a roll depressing force P, and 8 is a plate thickness operational means for calculating the thickness of the rolled material, under a condition in which roll separating forces are applied between the work rolls and between the back-up rolls, from the work roll separating force Q the back-up roll separating force Q the rolling pressure P, the modulus of rigidity K of the whole mill including the rolls and the frame, which modulus may be precalculated, the modulus of rigidity M the modulus of rigidity M and roll clearance setting S The numeral 9 shows a comparator having an output Ah which is the difference between the output h of the operational means 8 and a desired plate thickness h,,, and 10 is a roll clearance compensation computor for providing an output signal AS in accordance with the output Ah of the comparator 9 and introducing the signal AS into roll clearance adjusting means to provide a roll clearance adjustment AS. The numeral 11 shows a roll clearance adjusting means for moving a pressing screw 12 in accordance with the output of the roll clearance compensation computor 10, 13 is a material being rolled, and 14 is shape control means.

In accordance with the command from the shape control means 14, the outputs Q and Q of the force applying means 4 and 6 respectively are applied to the corresponding rolls/The roll separating forces Q and Q are respectively detected by the work roll separating force detector 3 and the back-up roll separating force detector 5. Further, the roll depressing force P is detected by the detector 7.

The detected values Q Q and P are then introduced into the plate thickness operational means 8 together with the pre-calculated modulus of rigidity K of the whole mill including the rolls and the frame, the modulus of rigidity M of the work and back-up rolls and the housing, and the modulus of rigidity M of the back-up roll and the housing to calculate the plate thickness h in the operational means 8. The comparator 9 calculates the difference Ah between the calculated thickness h and the desired thickness h and the value Ah is introduced into the roll clearance compensation computor 10. The output AS of the computor 10 is used to actuate the roll clearance adjusting means 11 to cause the movement of the pressing screw 12 until the roll clearance becomes to such a value that provides the desired plate thickness. In other words, the roll clearance is readjusted by an amount AS. Thus, according to the control system, in addition to improving the shape of the rolled material, any variation in the plate thickness is detected to perform a control for obviating the variation, so that the plate thickness can be controlled with a high accuracy.

The present invention can also be embodied in the form as shown in FIG. 7. In this drawing, corresponding parts and designated by the same reference numerals as in FIG. 6. The numeral 10' shows roll separating force compensation computor having outputs AQw and AQ respectively applied to means 4 for applying a work roll separating force O which is required for readjusting the roll clearance by an amount AS in accordance with the output Ah of the comparator 9, and to means 6 for applying a back-up roll separating force O and 14 shows roll separating force apply command means. In operation, the force applying means 4 and 6 applies their output forces O and Q respectively, which are then detected by the work roll separating force detector 3 and the back-up roll detector 5. The roll depressing force P is also detected by the roll depressing force detector 7. Further, the precalculated modulus of rigidity K of the whole mill including the rolls and the housing, the modulus of rigidity M of the work rolls, the back-up rolls and the housing, and the modulus of rigidity M of the back-up rolls and the housing are used together with the detected work roll separating force O the back-up roll separating force 0 the roll clearance setting S and the roll depressing force P to calculate the thickness h of the rolled material by the operational means 8 under the condition in which the roll separating forces and applied. The output of the operational means 8 is then introduced into the comparator 9 in which the difference Ah between the actual thickness h and the desired thickness h,,. The difference Ah is used in the roll separating force compensating computor 10 to obtain the roll separating force correction factors AQ and AQ which are then introduced into the command means 14. Thus, the command from the means 14' is changed so that the roll separating forces Q and QB are changed by the amount AQ and AQB, whereby theoperations of the force applying means 4 and 6 are varied by amounts corresponding to AQ and AQ respectively. This change in the roll separating forces is effective to readjust the roll clearance so that a rolled material of desired thickness can be obtained.

In this case, any change in thickness and/or hardeness of a material being rolled will cause a change of the roll depressing force P so that, by correcting the roll separating forces O and 0,, in accordance with the change of the roll depressing force P, a thickness substantially equal to the desired value h can be obtained. This control system is advantageous in that it can be designed, for example by means of hydraulic servo system, so as to respond very rapidly. Thus, even when there is any condition change in a material to be rolled, the change can be immediately compensated so that substantially constant plate thickness can be obtained.

The present invention can also be practiced by a combination of methods explained with reference to FIGS. 6 and 7. Further, it will be sufficient to apply either one of the work roll or back-up roll separating force.

A further embodiment shown in FIG. 8 will now be described. In this drawing, the same reference numerals designate the corresponding parts. The reference numeral 15 shows a crown detector for detecting the amount of crown on a plate material, and 16 shows a roll separating force corrector which is operated for controlling the roll separating force, when there is any deviation to the corrected in the crown shape, so as to eliminate the deviation. The numeral 17 shows a plate thickness detector. In the drawing, the plate thickness detector 17 and the crown detector 15 are disposed at the exit side of the roll stand. In a rolling operation, the work rolls and the back-up rolls are applied with the roll separating forces Q and 0,, by shape control means 14. The plate thickness operational means 8 calculates the plate thickness h at the exit side of the roll stand under a condition in which the roll separating forces are applied, in accordance with the equation (7) by using the factors O Q S K, M and M The output of the operational means 8 is introduced into the comparator 9. As in the embodiment explained with reference to FIG. 6, the plate thickness can thus be controlled. The crown detector 15 may comprise a plurality of plate thickness detectors arranged in a row extending widthwise of the plate to obtain the amount of crown by the difference among each of the detected values of the plate thickness detectors. Thus, the crown detector 15 detects the amount of crown which is then introduced into the roll separating force corrector 16. This corrector 16 produces outputs corresponding to the correction factors A0 and A0,, for adjusting the roll separating forces in accordance with the amount of crown. By the outputs AQ and AQB, the shape control means 14 is operated to adjust the forces Q and Q Further, AQ and AQ are relayed through the separating force detectors 3 and 5 to the plate thickness 0perational means 8. The operational means 8 calculates the plate thickness taking the factors A0 and AQ of the roll separating forces into account, and sends its output to the comparator 9. In the comparator 9, the actual thickness is compared with the desired thickness I1 and the roll clearance adjusting means 11 is actuated by the output of the computor to adjust the roll clearance through the depressing means 12 so as to eliminate the difference Ah. Thus, both the plate thickness and the shape can be controlled in a favourable manner during rolling operation. Further, in order to improve the accuracy, there is provided a plate thickness detector 17 to obtain the plate thickness h, at the exit side which is introduced into the operational means 8 by a negative feed back connection. The operational means 8 send the thickness it into the compara tor 9 when the output 12 of the plate thickness detector 17 is not equal to the desired value h,,, and the gain of the means is adjusted so that the calculated value h becomes equal to the output h, of the plate thickness detector 17.

According to the present invention, a crown detector may be located in front of rolling mill, and roll separating forces applied at the roll stand may be adjusted so that the crown can be eliminated at the exit side of the rolling mill, calculating the plate thickness at the exit side of the stand taking the adjustment of the roll separating force into account to readjust the roll clearance for obtaining a desired plate thickness.

As described above, according to the present invention, roll separating forces are applied for the purpose of shape control and a control is performed to eliminate plate thickness variations through an accurate evaluation of the change in plate thickness due to the application of the roll separating forces, so that a highly accurate plate thickness control can be obtained.

We claim:

1. Apparatus for controlling the thickness of a rolled material in a rolling mill of a type having a pair of work rolls and at least a pair of back up rolls wherein a roll separating force is applied between the roll bearings of the work rolls and between the roll bearings of the back up rolls ofa roll stand in the direction opposite to a roll depressing force, said apparatus comprising means for setting the roll clearance and for providing an output indicative thereof including roll depressing force control means; means for setting the roll separating force for the work roll bearings and for the back up roll bearings and for providing an output indicative thereof including roll separating force control means for the work roll bearings and for the back up roll bearings; setting means for providing an output indicative of the modulus of rigidity between the roll bearings and a housing and the modulus of rigidity of the whole mill; a detector for providing an output indicative of the roll depressing force; a detector for providing an output indicative of the roll separating force at the work roll bearings and a detector for providing an output indicative of the roll separating force at the back up roll bearings; plate thickness operational means responsive to the outputs of said setting means and detectors for providing an output indicative of the thickness of the rolled material on the exit side of the roll stand; a compressor for comparing the output of said operational means with a desired value to determine the deviation therebetween and providing an output indicative thereof; and control means for receiving the output of said comparator for generating a control signal to control at least one of said roll depressing force control means and said roll separating force control means for varying at least one of the depressing force and separating force at both the work roll bearings and back up roll bearings, and further comprising a detector for detecting the amount of crown of the rolled material on the exit side of the roll stand and shape control means for receiving the output of said crown detector and generating a signalto control said roll separating force control means for varying the separating force at the work and back up roll bearings so as to eliminate the crown.

2. Apparatus for controlling the thickness of a rolled material in a rolling mill of a type having a pair of work rolls and at least a pair of back up rolls wherein a roll separating force is applied between the roll bearings of the work rolls and between the roll bearings of the back up rolls of a roll stand in the direction opposite to a roll depressing force, said apparatus comprising means for setting the roll clearance and for providing an output indicative thereof including roll depressing force control means; means for setting the roll separating force for the work roll bearings and for the back up roll bearings and for providing an output indicative thereof including roll separating force control means for the work roll bearings and for the back up roll bearings; setting means for providing an output indicative of the modulus of rigidity between the roll bearings and a housing and the modulus of rigidity of the whole mill; a detector for providing an output indicative of the roll depressing force; a detector for providing an output indicative of the roll separating force at the work roll bearings and a detector for providing an output indicative of the roll separating force at the back up roll bearings; plate thickness operational means responsive to the outputs of said setting means and detectors for providing an output indicative of the thickness of the rolled material on the exit side of the roll stand; a comparator for comparing the output of said operational means with a desired value to determine the deviation therebetween and providing an output indicative thereof; and control means for receiving the output of said comparator for generating a control signal to control at least one of said roll depressingforce control means and said roll separating force control means for varying at least one of the depressing force and separating force at both the work roll bearings and back up roll bearings, and a thickness detector provided on the exit side of the roll stand and the roll clearance setting for the next rolling operation being corrected by the output of said thickness detector.

3. Apparatus for controlling the thickness of a rolled material in a rolling mill of a type having a pair of work rolls and at least a pair of back up rolls wherein a roll separating force is applied between the roll bearings of the work rolls and between the roll bearings of the back up rolls of a roll stand in the direction opposite to a roll depressing force. said apparatus comprising means for setting the roll clearance and for providing an output indicative thereof including roll depressing force control means; means for setting the roll separating force for the work roll bearings and for the back up roll bearings and for providing an output indicative thereof including roll separating force control means for the work roll bearings and for the back up roll bearings; setting means for providing an output indicative of the modulus of rigidity between the roll bearings and a housing and the modulus of rigidity of the whole mill; a detector for providing an output indicative of the roll depressing force; a detector for providing an output indicative of the roll separating force at the work roll bearings and a detector for providing an output indicative of the roll separating force at the back up roll bearings; plate thickness operational means responsive to the outputs of said setting means and detectors for providing an output indicative of the thickness of the rolled material on the exit side of the roll stand; a comparator for comparing the output of said operational means with a desired value to determine the deviation therebetween and providing an output indicative thereof; and control means for receiving the output of said comparator for generating a control signal to control at least one of said roll depressing force control means and said roll separating force control means for varying at least one of the depressing force and separating force at both the work roll bearings and back up roll bearings, and further comprising a plurality of thickness detectors provided in the widthwise direction of the material being rolled to detect the amount of crown of the material on the exit side of the roll stand, means for calculating the amount of crown from the output difference'of said thickness detectors and means for applying the output of said crown calculator means to said control means to control said roll depressing force control means and said roll separating force control means.

4. Apparatus for controlling the thickness of a rolled material in a rolling mill of a type having a pair of work rolls and at least a pair of back up rolls wherein a roll separating force is applied between the roll bearings of the work rolls and between the roll bearings of the back up rolls of a roll stand in the direction opposite to a roll depressing force, said apparatus comprising means for setting the roll clearance and for providing an output indicative thereof including roll depressing force control means; means for setting the roll separating force for the work roll bearings and for the back up roll bearings and for providing an output indicative thereof including roll separating force control means for the work roll bearings and for the back up roll bearings; setting means for providing an output indicative of the modulus of rigidity between the roll bearings and a housing and the modulus of rigidity of the whole mill; a detector for providing an output indicative of the roll depressing force; a detector for providing an output indicative of the roll separating force at the work roll bearings and a detector for providing an output indicative of the roll separating force at the back up roll bearings; plate thickness operational means responsive to the outputs of said setting means and detectors for providing an output indicative of the thickness of the rolled material on the exit side of the roll stand; a comparator means for comparing the output of said operational means with a desired value to determine the deviation therebetween and providing an output indicative thereof; and control means for receiving the output of said comparator means for generating a control signal to control at least one of said roll depressing force control means and said roll separating force control means for varying at least one of the depressing force and separating force at both the work roll bearings and back up roll bearings, said control means controlling said roll depressing force means in response to the output of said comparator means. 

1. Apparatus for controlling the thickness of a rolled material in a rolling mill of a type having a pair of work rolls and at least a pair of back up rolls wherein a roll separating force is applied between the roll bearings of the work rolls and between the roll bearings of the back up rolls of a roll stand in the direction opposite to a roll depressing force, said apparatus comprising means for setting the roll clearance and for providing an output indicative thereof including roll depressing force control means; means for setting the roll separating force for the work roll bearings and for the back up roll bearings and for providing an output indicative thereof including roll separating force control means for the work roll bearings and for the back up roll bearings; setting means for providing an output indicative of the modulus of rigidity between the roll bearings and a housing and the modulus of rigidity of the whole mill; a detector for providing an output indicative of the roll depressing force; a detector for providing an output indicative of the roll separating force at the work roll bearings and a detector for providing an output indicative of the roll separating force at the back up roll bearings; plate thickness operational means responsive to the outputs of said setting means and detectors for providing an output indicative of the thickness of the rolled material on the exit side of the roll stand; a compressor for comparing the output of said operational means with a desired value to determine the deviation therebetween and providing an output indicative thereof; and control means for recedving the output of said comparator for generating a control signal to control at least one of said roll depressing force control means and said roll separating force control means for varying at least one of the depressing force and separating force at both the work roll bearings and back up roll bearings, and further comprising a detector for detecting the amount of crown of the rolled material on the exit side of the roll stand and shape control means for receiving the output of said crown detector and generating a signal to control said roll separating force control means for varying the separating force at the work and back up roll bearings so as to eliminate the crown.
 2. Apparatus for controlling the thickness of a rolled material in a rolling mill of a type having a pair of work rolls and at least a pair of back up rolls wherein a roll separating force is applied between the roll bearings of the work rolls and between the roll bearings of the back up rolls of a roll stand in the direction opposite to a roll depressing force, said apparatus comprising means for setting the roll clearance and for providing an output indicative thereof including roll depressing force control means; means for setting the roll separating force for the work roll bearings and for the back up roll beaRings and for providing an output indicative thereof including roll separating force control means for the work roll bearings and for the back up roll bearings; setting means for providing an output indicative of the modulus of rigidity between the roll bearings and a housing and the modulus of rigidity of the whole mill; a detector for providing an output indicative of the roll depressing force; a detector for providing an output indicative of the roll separating force at the work roll bearings and a detector for providing an output indicative of the roll separating force at the back up roll bearings; plate thickness operational means responsive to the outputs of said setting means and detectors for providing an output indicative of the thickness of the rolled material on the exit side of the roll stand; a comparator for comparing the output of said operational means with a desired value to determine the deviation therebetween and providing an output indicative thereof; and control means for receiving the output of said comparator for generating a control signal to control at least one of said roll depressing force control means and said roll separating force control means for varying at least one of the depressing force and separating force at both the work roll bearings and back up roll bearings, and a thickness detector provided on the exit side of the roll stand and the roll clearance setting for the next rolling operation being corrected by the output of said thickness detector.
 3. Apparatus for controlling the thickness of a rolled material in a rolling mill of a type having a pair of work rolls and at least a pair of back up rolls wherein a roll separating force is applied between the roll bearings of the work rolls and between the roll bearings of the back up rolls of a roll stand in the direction opposite to a roll depressing force, said apparatus comprising means for setting the roll clearance and for providing an output indicative thereof including roll depressing force control means; means for setting the roll separating force for the work roll bearings and for the back up roll bearings and for providing an output indicative thereof including roll separating force control means for the work roll bearings and for the back up roll bearings; setting means for providing an output indicative of the modulus of rigidity between the roll bearings and a housing and the modulus of rigidity of the whole mill; a detector for providing an output indicative of the roll depressing force; a detector for providing an output indicative of the roll separating force at the work roll bearings and a detector for providing an output indicative of the roll separating force at the back up roll bearings; plate thickness operational means responsive to the outputs of said setting means and detectors for providing an output indicative of the thickness of the rolled material on the exit side of the roll stand; a comparator for comparing the output of said operational means with a desired value to determine the deviation therebetween and providing an output indicative thereof; and control means for receiving the output of said comparator for generating a control signal to control at least one of said roll depressing force control means and said roll separating force control means for varying at least one of the depressing force and separating force at both the work roll bearings and back up roll bearings, and further comprising a plurality of thickness detectors provided in the widthwise direction of the material being rolled to detect the amount of crown of the material on the exit side of the roll stand, means for calculating the amount of crown from the output difference of said thickness detectors and means for applying the output of said crown calculator means to said control means to control said roll depressing force control means and said roll separating force control means.
 4. Apparatus for controlling the thickness of a rolled material in a rolling mill of a type having a pair of woRk rolls and at least a pair of back up rolls wherein a roll separating force is applied between the roll bearings of the work rolls and between the roll bearings of the back up rolls of a roll stand in the direction opposite to a roll depressing force, said apparatus comprising means for setting the roll clearance and for providing an output indicative thereof including roll depressing force control means; means for setting the roll separating force for the work roll bearings and for the back up roll bearings and for providing an output indicative thereof including roll separating force control means for the work roll bearings and for the back up roll bearings; setting means for providing an output indicative of the modulus of rigidity between the roll bearings and a housing and the modulus of rigidity of the whole mill; a detector for providing an output indicative of the roll depressing force; a detector for providing an output indicative of the roll separating force at the work roll bearings and a detector for providing an output indicative of the roll separating force at the back up roll bearings; plate thickness operational means responsive to the outputs of said setting means and detectors for providing an output indicative of the thickness of the rolled material on the exit side of the roll stand; a comparator means for comparing the output of said operational means with a desired value to determine the deviation therebetween and providing an output indicative thereof; and control means for receiving the output of said comparator means for generating a control signal to control at least one of said roll depressing force control means and said roll separating force control means for varying at least one of the depressing force and separating force at both the work roll bearings and back up roll bearings, said control means controlling said roll depressing force means in response to the output of said comparator means. 